What Happens to the Patellar Tendon After BPTB ACL Reconstruction
The patellar tendon undergoes compensatory enlargement through scar tissue formation after bone-patellar tendon-bone (BPTB) harvest for ACL reconstruction, which allows for recovery of tendon stiffness despite the newly formed tissue having inferior mechanical properties. 1
Anatomical Changes
- After harvesting the central third of the patellar tendon for BPTB autograft, the donor site undergoes a healing process that results in a 21% larger cross-sectional area compared to normal tendons 1
- The harvested area fills with scar tissue that has different mechanical properties than the original tendon tissue 1
- Despite the increased cross-sectional area, the Young's modulus (material quality) is reduced by approximately 24% in the reconstructed tendon 1
Functional Recovery
- The compensatory enlargement of the patellar tendon allows for recovery of overall tendon stiffness, enabling functional restoration 1
- Rehabilitation protocols typically allow for closed kinetic chain exercises starting from week 2 post-operation, with careful progression to open kinetic chain exercises by week 4 2
- For BPTB grafts, open kinetic chain exercises can be started from 4 weeks postoperative in a restricted range of motion (90°-45°) with gradual progression to full range of motion by week 8 2
Potential Complications
Donor Site Morbidity
- Anterior knee pain is a frequent complication after BPTB autograft harvest, particularly with kneeling activities 3
- Donor site morbidity must be balanced against the benefits of BPTB grafts, which have advantages in reducing graft failure risk compared to hamstring grafts 4
Risk of Patellar Fracture
- Patellar fracture is a rare but significant complication following BPTB harvest for ACL reconstruction 5
- Fractures can occur during rehabilitation, particularly during eccentric quadriceps exercises or maximum voluntary isometric contractions 5
- The risk of fracture persists long-term - if a patient later requires total knee arthroplasty (TKA), there is a significantly lower maximum load to failure of a resurfaced patella in the presence of a previous BPTB autograft 6
Patellar Tendon Rupture
- Complete rupture of the patellar tendon is a rare complication after BPTB harvest 7
- When ruptures occur, they typically present with an atypical tear pattern that can be technically challenging to address surgically 7
Rehabilitation Considerations
- Rehabilitation protocols must balance the competing goals of improving quadriceps strength while protecting both the healing graft and the patellar donor site 5
- Neuromuscular training should be added to strength training to optimize outcomes and prevent reinjuries 2
- An extensive test battery should be used to determine readiness for return to play, including strength tests, hop tests, and quality of movement assessments 2
- For pivoting/contact sports, a Limb Symmetry Index (LSI) of 100% is recommended before return to play 2
Long-term Outcomes
- Despite initial donor site morbidity, the patellar tendon typically regains functional stiffness within 1-10 years after harvest 1
- However, the altered mechanical properties of the healed tendon may contribute to long-term complications such as increased risk of patellar fracture during subsequent knee surgeries 6
- When patients with previous BPTB autograft later undergo total knee arthroplasty, there is approximately 37% reduction in the maximum load to failure of the resurfaced patella 6